Stop insert pins associated with collet chucks are old in the art but normally are used as a lathe stop (see U.S. Pat. No. 2,830,821) for longitudinal adjustment of the stop head. Other stop pins are known in the art but not used in the manner employed in the instant invention unless by use of complicated mechanisms such as shown in U.S. Pat. No. 2,830,821 employing heavy threaded collars.